1. Field of the Invention
This invention relates to a gasket suitable for spanning a joint between two adjacent floor sections, and more particularly to a flat expansion joint gasket that is resistant to upward bowing under compression.
2. Description of the Prior Art
Expansion joints are commonly used between floor sections in both load bearing and non-load bearing applications. Usually, a compressible gasket is used to cover the joint and to maintain a flat surface between the floor sections. These gaskets traverse a joint and frequently are required to bear a load.
Prior art gaskets designed for use in such applications are known. U.S. Pat. No. 5,048,249 describes an expansion joint floor covering having a cellular core structure made up of a multiplicity of longitudinally extending cells defined by transversely spaced-apart generally vertical walls and vertically spaced apart upper and lower walls interconnecting the top and bottom edges of the vertical walls. The bottom of each cell is preferably formed in two upwardly converging angular wall sections which are adapted to fold in accordion-like fashion to accommodate transverse expansion and compression. The upper wall has a slightly concave surface when the gasket is in the relaxed, or uncompressed shape. In practice, it has been found that gaskets of this or similar construction can still bow upward to an unacceptable extent. It is believed that the pleated and relatively compressible lower wall of this gasket tends to preferentially absorb the initial lateral compression force, making it shorter in length than the upper wall. Because the upper wall is comparatively thick and less compressible, it tends to initially bow upward as a unit. The upper wall is then no longer parallel to the direction of the applied lateral compressive force, so the application of further compressive force tends not to compress the top wall, but rather to bend it further, unlike the bottom wall. Aside from presenting a displeasing appearance, such a deformed gasket can easily be damaged by traditional floor cleaning equipment, and also presents a tripping hazard.
A transverse cross section of a prior art gasket 10 is illustrated in FIG. 1(A) and (B). The nominal (i.e., unstretched and uncompressed) gasket is shown in FIG. 1(A). This gasket comprises an upper wall 2, a plurality of vertical members 4, and a bottom wall 6 including the bottom surface of vertical members 4 and upwardly directed folds 8. Voids 14, which are essentially rectangular but for the upward folds 8, are formed between the upper wall 2, vertical members 4, and folds 8. An attachment means comprising a horizontal extension 16 and a longitudinally elongate barbed flap 12 depending downwardly therefrom is provided at each side of gasket 10 to attach the gasket to a joint. Folds 8 have lower strength than the material used for upper wall 2 to facilitate the compression of the gasket.
As shown in FIG. 1(B), upward folds 8, having lesser strength than upper wall 2, tend to absorb the initial compressive force when a joint in which the gasket sits contracts. This compression collapses of voids 14. Ideally, upper wall 2 should compress by the same amount of bottom wall 6 if the gasket is to remain flat. However, this never happens in practice. Without wishing to be limited to any particular explanation of the phenomenon, it is believed that the compression of the gasket inevitably creates vertically directed forces. If these vertical forces are communicated to the upper wall 2, the upper wall will bend in response thereto. If the pleats 8 on the lower wall fold inward, as they immediately will when compressed, vertical forces will tend to be communicated to the upper wall. Moreover, upper wall 2 is much more readily bent as a unit than compressed, and once bent, lateral compressive forces will tend to bend it further rather than compress it. As the length of the bottom wall 6 decreases because of the upwardly directed folding, and upper wall 2, which is bent rather than compressed, retains substantially the same length, upper wall 2 necessarily bows upward and outward from the joint. Although the upward bowing may be masked somewhat by providing upper wall 2 with a concave upper surface, a nominally flat, horizontal surface is preferable for many applications. Moreover, the extent of the bowing is great enough to be difficult to mask effectively in this manner for joints of moderate nominal width.
There is thus a need for an expansion joint gasket that can be compressed without excessive upward bowing and which has a nominally flat upper wall surface. In addition, it would be desirable to provide a single gasket for use in both load-bearing and nonload-bearing applications.